Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 231: Which of the following statements is true regarding GLUT-5?

A. Present in the brain
B. Present in adipose tissue, skeletal muscle, and skin
C. Mediated by insulin transport
D. Sodium independent transport (Correct Answer)

Explanation: **Explanation:** Glucose transporters (GLUTs) are a family of membrane proteins that facilitate the transport of glucose and other sugars across the cell membrane via **facilitated diffusion**. This process is **sodium-independent**, unlike the SGLT (Sodium-Glucose Linked Transporters) found in the kidneys and intestines, which require a sodium gradient. **Why Option D is correct:** GLUT-5 is unique among the GLUT family because its primary substrate is **fructose**, not glucose. Like all members of the GLUT family (GLUT 1-14), it operates via facilitated diffusion, meaning it moves solutes down their concentration gradient without requiring energy or sodium ions. **Analysis of Incorrect Options:** * **Option A:** GLUT-1 and GLUT-3 are the primary transporters in the **brain**, ensuring a constant glucose supply regardless of blood sugar levels. * **Option B:** GLUT-4 is the specific transporter found in **adipose tissue and skeletal muscle**. * **Option C:** Only **GLUT-4** is insulin-dependent. GLUT-5 is insulin-independent and is primarily expressed in the small intestine (apical membrane of enterocytes), spermatozoa, and kidneys. **High-Yield NEET-PG Pearls:** * **GLUT-1:** Blood-brain barrier, RBCs (Basal uptake). * **GLUT-2:** Bidirectional; found in Liver, Pancreas (B-cells), and Kidney. It has a high $K_m$ (low affinity). * **GLUT-3:** Neurons (Highest affinity). * **GLUT-4:** Insulin-responsive (Muscle/Fat). * **GLUT-5:** Fructose specific. * **SGLT-1/2:** Secondary active transport (Sodium-dependent); found in the small intestine and proximal convoluted tubule of the kidney.

Question 232: In a well-fed state, what is the major fate of glucose-6-phosphate in tissues?

A. Hydrolysis to glucose
B. Conversion to glycogen (Correct Answer)
C. Isomerization to fructose-6-phosphate
D. Conversion to ribulose-5-phosphate

Explanation: **Explanation:** In a **well-fed state**, high blood glucose levels trigger the release of **insulin**. Insulin promotes anabolic pathways to store excess energy. Glucose entering the cell is immediately phosphorylated to **Glucose-6-Phosphate (G6P)** by hexokinase/glucokinase. In tissues like the liver and muscle, the primary fate of this G6P is storage. It is converted to Glucose-1-Phosphate and then incorporated into **glycogen** via glycogen synthase (the rate-limiting enzyme activated by insulin). This ensures a glucose reserve for future fasting states. **Analysis of Incorrect Options:** * **Option A (Hydrolysis to glucose):** This occurs primarily in the liver during the **fasting state** (via Glucose-6-Phosphatase) to maintain blood glucose levels. In a well-fed state, this reaction is inhibited. * **Option C (Isomerization to fructose-6-phosphate):** While this is the second step of glycolysis, in a well-fed state, the "major fate" for storage takes precedence once immediate energy needs are met. Glycolysis provides ATP, but glycogen synthesis is the hallmark of the "excess" glucose state. * **Option D (Conversion to ribulose-5-phosphate):** This is the Pentose Phosphate Pathway (PPP). While active in the well-fed state (to provide NADPH for fatty acid synthesis), it is a minor pathway compared to the bulk storage of glucose as glycogen. **NEET-PG High-Yield Pearls:** * **Glucokinase vs. Hexokinase:** Glucokinase (liver/pancreas) has a high $K_m$ and high $V_{max}$, allowing it to handle the large glucose load in the well-fed state. * **Rate-limiting enzyme:** Glycogen synthase is active when **dephosphorylated** (stimulated by insulin). * **Tissue Specificity:** Muscle glycogen is used for local energy; only liver glycogen can contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**.

Question 233: Glucose can be synthesized from which of the following substrates?

A. Glycerol (Correct Answer)
B. Adenine
C. Guanine
D. Palmitic acid

Explanation: **Explanation:** The process of synthesizing glucose from non-carbohydrate precursors is known as **Gluconeogenesis**. This metabolic pathway primarily occurs in the liver and kidneys during periods of fasting or starvation. **Why Glycerol is Correct:** Glycerol is derived from the hydrolysis of triacylglycerols (TAGs) in adipose tissue. Once released, it is transported to the liver where it is phosphorylated by **glycerol kinase** to glycerol-3-phosphate and then oxidized to **Dihydroxyacetone phosphate (DHAP)**. DHAP is a direct intermediate of the glycolytic/gluconeogenic pathway, allowing glycerol to enter the pathway and be converted into glucose. **Why the other options are Incorrect:** * **Adenine & Guanine (Options B & C):** These are purine bases. The catabolism of purines in humans results in **uric acid**, which is excreted in the urine. Purines do not possess a carbon skeleton that can be converted into pyruvate or any TCA cycle intermediate; therefore, they cannot contribute to gluconeogenesis. * **Palmitic Acid (Option D):** This is a long-chain even-carbon fatty acid. Beta-oxidation of even-chain fatty acids yields **Acetyl-CoA**. In humans, Acetyl-CoA cannot be converted back to pyruvate because the **Pyruvate Dehydrogenase (PDH) reaction is irreversible**. Furthermore, the two carbons of Acetyl-CoA are lost as $CO_2$ in the TCA cycle, resulting in no net synthesis of glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Major Gluconeogenic Precursors:** Lactate (Cori Cycle), Glucogenic amino acids (primarily Alanine), and Glycerol. * **Odd-chain Fatty Acids:** Unlike Palmitic acid, odd-chain fatty acids *can* be gluconeogenic because their final breakdown product is **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * **Key Enzyme:** Glycerol kinase is absent in adipose tissue; therefore, glycerol must travel to the liver for gluconeogenesis.

Question 234: What is the alternative oxidative pathway for glucose?

A. Glycogenolysis
B. Gluconeogenesis
C. Glycogenesis
D. Glucuronic acid pathway (Correct Answer)

Explanation: **Explanation:** The **Glucuronic acid pathway** (also known as the Uronic acid pathway) is an **alternative oxidative pathway** for glucose that does not lead to the generation of ATP. While the primary oxidative pathway for glucose is Glycolysis (followed by the TCA cycle), the Glucuronic acid pathway serves specialized functions. It converts Glucose-6-phosphate into **Glucuronic acid**, which is essential for the conjugation and detoxification of bilirubin, steroid hormones, and various drugs in the liver. It is also the precursor for the synthesis of Vitamin C (ascorbic acid) in most animals, though humans lack the enzyme *L-gulonolactone oxidase* to complete this synthesis. **Analysis of Incorrect Options:** * **A. Glycogenolysis:** This is the process of breaking down glycogen into glucose-1-phosphate; it is a catabolic pathway but not an oxidative pathway for glucose itself. * **B. Gluconeogenesis:** This is the synthesis of glucose from non-carbohydrate precursors (like lactate or glycerol); it is a synthetic (anabolic) pathway. * **C. Glycogenesis:** This is the process of glycogen synthesis from glucose for storage; it is an anabolic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Pentosuria:** A rare, benign genetic deficiency of the enzyme **Xylitol dehydrogenase** in this pathway leads to the excretion of L-xylulose in the urine. * **Detoxification:** Glucuronic acid is conjugated with bilirubin to form **Bilirubin Diglucuronide** (conjugated bilirubin), making it water-soluble for excretion. * **Drug Metabolism:** Many drugs (e.g., morphine, paracetamol) are excreted as glucuronides via this pathway.

Question 235: The branching enzyme is involved in which process?

A. Glycogenesis (Correct Answer)
B. Glucogenesis
C. Glycogenolysis
D. Glycolysis

Explanation: **Explanation:** **1. Why Glycogenesis is Correct:** Glycogenesis is the process of glycogen synthesis. The **Branching Enzyme** (also known as **Amylo-(1,4→1,6)-transglucosidase**) is essential for creating the branched structure of glycogen. Once glycogen synthase extends a glucose chain via α-1,4-glycosidic bonds to about 11 residues, the branching enzyme removes a fragment (at least 6-7 residues) and reattaches it via an **α-1,6-glycosidic bond**. Branching increases the solubility of glycogen and creates multiple non-reducing ends, allowing for rapid mobilization of glucose during stress or fasting. **2. Why Other Options are Incorrect:** * **Glucogenesis:** This is a general term for glucose formation (often confused with Gluconeogenesis). It does not involve branching enzymes. * **Glycogenolysis:** This is the breakdown of glycogen. It requires the **Debranching Enzyme** (which has two activities: 4:4 transferase and α-1,6-glucosidase) to remove branches, not create them. * **Glycolysis:** This is the metabolic pathway that converts glucose into pyruvate. It involves enzymes like Hexokinase and Phosphofructokinase-1; it does not involve glycogen or branching. **3. High-Yield Clinical Pearls for NEET-PG:** * **Andersen Disease (GSD Type IV):** Caused by a deficiency of the **Branching Enzyme**. It results in the formation of abnormal glycogen with very long outer chains (resembling amylopectin), leading to liver cirrhosis and early death. * **Cori Disease (GSD Type III):** Caused by a deficiency of the **Debranching Enzyme**, leading to the accumulation of "limit dextrins." * **Key Regulatory Enzyme:** Remember that **Glycogen Synthase** is the rate-limiting enzyme for glycogenesis, but the Branching Enzyme is required for structural integrity.

Question 236: Heparin is a type of:

A. Polysaccharide (Correct Answer)
B. Lipoprotein
C. Monosaccharide
D. Polyenoic acid

Explanation: **Explanation:** Heparin is a highly sulfated glycosaminoglycan (GAG), which is a specific class of **heteropolysaccharides**. It consists of repeating disaccharide units—specifically, D-glucosamine and either L-iduronic acid or D-glucuronic acid. Because it is composed of long chains of multiple sugar units, it is classified as a polysaccharide. **Analysis of Options:** * **A. Polysaccharide (Correct):** Heparin is a complex carbohydrate. It is the most acidic substance in the human body due to its high sulfate and carboxyl group content, which gives it a strong negative charge. * **B. Lipoprotein:** These are complexes of lipids and proteins (e.g., LDL, HDL) used for lipid transport. Heparin is not a lipid-based molecule, though it does interact with Lipoprotein Lipase (LPL) to clear triglycerides from the blood. * **C. Monosaccharide:** These are simple sugars (e.g., glucose, fructose) that cannot be hydrolyzed further. Heparin is a large polymer, not a single sugar unit. * **D. Polyenoic acid:** This term refers to polyunsaturated fatty acids (PUFAs) containing multiple double bonds (e.g., Linoleic acid). Heparin is a carbohydrate, not a fatty acid. **Clinical Pearls for NEET-PG:** * **Mechanism:** Heparin acts by binding to **Antithrombin III**, increasing its affinity for thrombin and Factor Xa by 1000-fold. * **Location:** It is primarily found in the secretory granules of **mast cells**. * **Antidote:** The strong negative charge of heparin is neutralized by the positively charged **Protamine Sulfate**. * **Diagnostic Use:** It is the preferred anticoagulant for blood gas analysis and pH estimation.

Question 237: Which of the following enzymes does not participate in the TCA cycle?

A. Citrate synthase
B. Isocitrate dehydrogenase
C. Pyruvate dehydrogenase (Correct Answer)
D. Malate dehydrogenase

Explanation: **Explanation:** The **Tricarboxylic Acid (TCA) cycle**, also known as the Krebs cycle, occurs in the mitochondrial matrix. It begins with the condensation of Acetyl-CoA and Oxaloacetate. **Why Pyruvate Dehydrogenase (PDH) is the correct answer:** Pyruvate dehydrogenase is part of the **PDH Complex**, which serves as a "bridge" or "link reaction" between glycolysis and the TCA cycle. It converts Pyruvate (the end product of glycolysis) into Acetyl-CoA. While it provides the substrate necessary for the cycle to begin, it is **not** considered a member of the TCA cycle itself. **Analysis of Incorrect Options:** * **Citrate Synthase (Option A):** This is the first regulatory enzyme of the TCA cycle. It catalyzes the synthesis of Citrate from Acetyl-CoA and Oxaloacetate. * **Isocitrate Dehydrogenase (Option B):** This is the **rate-limiting enzyme** of the TCA cycle. It catalyzes the oxidative decarboxylation of Isocitrate to α-Ketoglutarate, producing the first molecule of NADH and $CO_2$. * **Malate Dehydrogenase (Option D):** This is the final enzyme of the cycle, which oxidizes Malate to Oxaloacetate, completing the loop and generating NADH. **High-Yield Clinical Pearls for NEET-PG:** * **PDH Deficiency:** The most common cause of congenital lactic acidosis. * **Cofactors:** Both PDH and α-Ketoglutarate dehydrogenase require five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**eves **N**ever **P**lay **L**ate). * **Inhibitors:** Fluoroacetate inhibits Aconitase; Arsenite inhibits α-Ketoglutarate dehydrogenase. * **ATP Yield:** One turn of the TCA cycle yields **10 ATP** (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1).

Question 238: Substrate-level phosphorylation occurs at which of the following enzymes?

A. Pyruvate kinase (Correct Answer)
B. Glucokinase
C. Glucose-6-phosphatase
D. alpha-ketoglutarate dehydrogenase

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, independent of the electron transport chain and oxygen. **Why Pyruvate Kinase is Correct:** In the final step of glycolysis, **Pyruvate Kinase** catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two sites of SLP in glycolysis (the other being Phosphoglycerate kinase). **Analysis of Incorrect Options:** * **B. Glucokinase:** This enzyme catalyzes the phosphorylation of glucose to glucose-6-phosphate. It **consumes** one molecule of ATP rather than generating it. * **C. Glucose-6-phosphatase:** This is a gluconeogenic enzyme that removes a phosphate group from glucose-6-phosphate to release free glucose. It does not involve ATP synthesis. * **D. Alpha-ketoglutarate dehydrogenase:** This is a multi-enzyme complex in the TCA cycle that produces NADH and $CO_2$. While the *next* step in the cycle (Succinate thiokinase) performs SLP to produce GTP, this specific enzyme does not. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP sites to remember:** 1. **Glycolysis:** Phosphoglycerate kinase and Pyruvate kinase. 2. **TCA Cycle:** Succinate thiokinase (Succinyl-CoA synthetase) – produces GTP. * **Clinical Correlation:** Pyruvate kinase deficiency is the most common cause of enzyme-deficient **hereditary non-spherocytic hemolytic anemia**. Without SLP, RBCs cannot maintain the Na+/K+ ATPase pump, leading to cell swelling and lysis. * **Inhibitor Note:** Arsenate can uncouple SLP at the glyceraldehyde-3-phosphate dehydrogenase step, resulting in zero net ATP gain from glycolysis.

Question 239: During caloric deprivation, how many hours are typically required for complete depletion of hepatic glycogen stores?

A. 9 hours
B. 18 hours (Correct Answer)
C. 24 hours
D. 48 hours

Explanation: **Explanation:** **1. Why 18 hours is correct:** Hepatic glycogen serves as the primary source of blood glucose during the early stages of fasting. In a healthy adult, the liver stores approximately 75–100 grams of glycogen. Following a meal, blood glucose levels are maintained by dietary intake for about 4 hours. As the body enters the post-absorptive state, **glycogenolysis** (breakdown of glycogen) becomes the dominant process. Under standard caloric deprivation, these stores are progressively depleted and typically run out within **12 to 18 hours**. By this point, the body must transition almost entirely to **gluconeogenesis** (synthesis of glucose from non-carbohydrate sources like amino acids and glycerol) to maintain glycemia. **2. Analysis of Incorrect Options:** * **9 hours (Option A):** At this stage, hepatic glycogenolysis is still at its peak. The stores are significantly reduced but not yet exhausted. * **24 hours (Option C):** While some textbooks suggest a range of 12–24 hours, 18 hours is the physiological "tipping point" emphasized in standard medical biochemistry (like Harper’s). By 24 hours, gluconeogenesis is already the sole source of blood glucose. * **48 hours (Option D):** This represents prolonged fasting. By 48 hours, the body has shifted toward ketosis to spare muscle protein. **3. NEET-PG High-Yield Pearls:** * **Muscle Glycogen:** Unlike the liver, muscle glycogen **cannot** contribute to blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**. * **Rate-Limiting Enzyme:** The rate-limiting enzyme for glycogenolysis is **Glycogen Phosphorylase**. * **Gluconeogenesis Trigger:** As glycogen levels fall, the **Insulin:Glucagon ratio** decreases, activating PEPCK and Fructose-1,6-bisphosphatase. * **Clinical Correlation:** In Von Gierke’s Disease (G6Pase deficiency), severe hypoglycemia occurs much earlier because neither glycogenolysis nor gluconeogenesis can release glucose into the blood.

Question 240: What is the enzyme deficiency in galactosemia?

A. Glucokinase
B. Alloblase
C. Galactokinase (Correct Answer)
D. All of the above

Explanation: **Explanation:** Galactosemia is a group of inherited metabolic disorders characterized by the body's inability to metabolize galactose. The metabolism of galactose occurs via the **Leloir pathway**. **Why Galactokinase is correct:** Galactokinase (GALK) is the first enzyme in the Leloir pathway, responsible for phosphorylating galactose into **galactose-1-phosphate**. A deficiency in this enzyme leads to **Galactokinase Deficiency (Type II Galactosemia)**. This condition is characterized by the accumulation of galactose in the blood (galactosemia) and urine (galactosuria). The excess galactose is diverted to the polyol pathway, where it is reduced to **galactitol** by aldose reductase, leading to the primary clinical feature: early-onset cataracts. **Why other options are incorrect:** * **Glucokinase:** This enzyme catalyzes the phosphorylation of glucose to glucose-6-phosphate, primarily in the liver and pancreas. It is not involved in galactose metabolism. * **Alloblase:** This is not a recognized enzyme in human carbohydrate metabolism. It appears to be a distractor term. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Galactosemia (Type I):** Caused by a deficiency of **GALT (Galactose-1-phosphate uridyltransferase)**. It is more severe than GALK deficiency, presenting with jaundice, hepatomegaly, E. coli sepsis, and intellectual disability. * **Type III Galactosemia:** Caused by **UDP-galactose-4-epimerase (GALE)** deficiency. * **Diagnostic Hallmark:** The presence of non-glucose reducing sugars in the urine (positive Benedict's test, negative glucose oxidase test). * **Management:** Immediate exclusion of lactose and galactose from the diet (e.g., switching to soy-based formula).

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

Practice Questions

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